Cumene is an important intermediate in the chemical and polymer industries, with global cumene production currently exceeding twelve million metric tons annually. The majority of all cumene manufactured in the world today is used for the production of phenol. The demand for phenol for the manufacture of Bisphenol-A and subsequently polycarbonates is accelerating, owing to the broadening applications of polycarbonates in the electronic, healthcare, and automobile industries.
The rapid growth of cumene, phenol and Bisphenol-A production, however, has caused some concerns related to the imbalance of the acetone byproduct produced from the phenol plant. Thus, acetone and phenol are produced at an approximately 1:1 molar ratio from cumene, but are used at an approximately 1:2 molar ratio in the downstream Bisphenol-A production process. The excess acetone that is not used in the production of Bisphenol-A has caused some concern from phenol producers in that it may create a supply-demand imbalance and disrupt the economics of the phenol production business.
In addition, cumene is typically produced by reacting benzene and propylene under liquid phase or mixed gas-liquid phase conditions in the presence of acid catalysts, particularly zeolite catalysts. The resultant need to locate integrated cumene/phenol plants near a source of propylene has become an important issue with producers. Thus, in today's olefins market, there is also a supply-demand imbalance in the supply of propylene produced from conventional sources, such as ethylene plants, due to the reduced availability of feedstocks that favor the production of propylene. This imbalance has forced phenol producers to build their plants closer to feedstock supplies rather than to product outlets.
Numerous research efforts have been directed at solving the acetone imbalance and propylene supply issues described above by employing the excess acetone produced in the phenol plant to produce cumene. For example, U.S. Pat. No. 5,015,786 teaches a process for preparing phenol, comprising the steps of: (a) alkylating benzene with isopropanol using a zeolite catalyst under liquid phase conditions to synthesize cumene, (b) oxidizing the cumene from step (a) with molecular oxygen into cumene hydroperoxide, (c) subjecting cumene hydroperoxide to acid cleavage to synthesize phenol and acetone, and (d) hydrogenating the acetone from step (c) with hydrogen gas under liquid phase conditions into isopropanol which is recycled to step (a).
One problem encountered in producing cumene from the excess acetone from a phenol plant is that the acetone tends to contain significant quantities of nitrogenous impurities which carry over into the isopropanol intermediate product. Such impurities act as poisons to the zeolite catalyst employed in the downstream alkylation step and so must be removed or reduced to very low levels. However, attempts to remove these impurities from the acetone and isopropanol feeds with conventional solid acid adsorbents have proved to be only marginally effective due to the molecular polarity of the acetone and isopropanol, which competes with the adsorption of the polar nitrogen compounds. Also, the high water solubility of acetone and isopropanol eliminates the use of water washing, which is also commonly employed to remove nitrogen compounds from hydrocarbon streams.
Despite these problems, various methods are being investigated for reducing the level of nitrogen impurities in the isopropanol feed to the zeolite alkylation catalyst. However, whatever method is employed, it is very difficult to reduce these levels to zero and so in practice the zeolite catalyst will have a limited cycle life owing to the build-up of nitrogen compounds at the active acid sites of the catalyst. There is therefore a need for a method of rejuvenating the spent catalyst from an isopropanol alkylation process so as to extend its cycle life.
U.S. Published Patent Application No. 2010/0285949 discloses a method for rejuvenating a catalyst, particularly a spent catalyst used in the alkylation of benzene with propylene to produce cumene. The catalyst comprises at least 10 wt. % of a molecular sieve selected from at least one of a MCM-22 family molecular sieve, a molecular sieve having a framework type of BEA, a molecular sieve having a framework type of FAU, and a molecular sieve having a framework type of MOR, wherein the spent catalyst also comprises from 0.001 wt. % to 45 wt. % of hydrocarbons and 0.001 to 10 wt. % nitrogen containing components. The rejuvenation method comprises contacting the spent catalyst with a gaseous feedstock comprising at least one of N2, H2, alkane, He, Ar, CO, and CO2 for at least one hour at rejuvenation conditions comprising a temperature in the range from about 400 to 600° C., a pressure in the range from about 101.3 kPa-a to 10130 kPa-a, a space hourly velocity in the range of from 0.05 to 10 normal cubic meter gaseous feedstock per hour per kilogram of catalyst to form a rejuvenated catalyst and a gaseous product. The rejuvenated catalyst comprises at least 50 wt. % less nitrogen containing components than the catalyst prior to the contacting step, and the gaseous product comprises at least a portion of the gaseous feedstock and at least a portion of the hydrocarbons and the nitrogen containing components contained by the spent catalyst.
In accordance with the present invention, it has now been found that a spent zeolite catalyst deactivated in the alkylation of benzene with isopropanol can be successfully rejuvenated by treatment with a gaseous stripping agent, such as of N2, H2, alkane, He, Ar, CO, and CO2. This is surprising since the spent catalyst contains nitrogenous contaminants, which originated in the phenol process and which are significantly different from the nitrogenous impurities deposited on the catalyst when used to produce cumene by alkylation of benzene with propylene, which are typically ammonia and amines. The nitrogenous impurities originating in the phenol process result from the use of steam condensate and caustic in washing steps in the phenol process Filming amines and nitrogenous corrosion inhibitors that are present in the steam condensate and caustic are transferred to cumene recycle streams in the phenol process. These components subsequently undergo chemical reaction in the oxidation and hydroperoxide cleavage steps of the phenol process creating highly polar and non-ideal components in the acetone product, such as imines and oximes. These impurities are substantially different in nature from the typical nitrogenous impurities present in the cumene process by alkylation of benzene and propylene.